1. Field of the Invention
This is invention is related to device and apparatus and methods for producing white light from luminescent particle excitation and emission.
2. Description of the Related Art
The choice of general illumination sources for commercial and residential lighting is generally governed by a balance of energy efficiency and the ability to faithfully produce colors as measured by the color rendering index (CRI). Existing fluorescent lighting is known to be economical from an energy consumption point of view. However, many users complain that the light produced by the existing fluorescent lighting is of poor spectral quality and produces eye strain and other adverse health effects. Incandescent light is also widely used and is recognized as having excellent spectral quality and the ability to accurately render colors. This high spectral quality is derived from the hot filament, which approximates a blackbody radiator and emits light over many wavelengths, similar to the sun. However, incandescent lighting suffers from very low energy efficiency. Thus, there is a long felt need to produce light sources that use less energy and have a light composition similar to the composition of the sun light.
Solid-state lighting (SSL) is an alternative general illumination and lighting technology that promises the energy efficiency of fluorescent lights and the excellent spectral qualities of incandescent lighting. Typically, commercially available SSL lamps consists of a light emitting diode (LED) surrounded by a phosphor composed of large particles usually larger than 2 μm. The light emitted from the LED is of sufficient energy to cause the phosphor to fluoresce and emit one or more colors of visible light. The most common example of commercial SSL products consists of a blue LED (typically 460 nm) surrounded by a yellow phosphor, such as cerium-doped yttrium aluminum garnet (YAG:Ce), that emits lights in a broad band centered at 550 nm. The combination of nominally yellow light emission from the phosphor and blue light from the LED produces a light source that has a generally white appearance. Alternatively, an LED that emits in the ultraviolet (<400 nm) can be used to excite a blend of red, green, and blue phosphors. FIG. 1 is a schematic depiction of the spectrum of light obtained from a solid-state lighting device. While this approach produces white light, it suffers from low efficiency and poor spectral quality due to the limited number of wavelengths.
In addition, while the light intensity from lamps used in current solid-state lighting products is sufficient for applications such as flashlights, it is considered too low and the emission cone is considered too narrow for use in general illumination applications such as room lighting. Hence, there is a need for solid-state light sources that are capable of providing high intensity white light emissions over a large enough area for use in general illumination.
One approach proposed to improve the performance of SSL devices has been to use nanoparticles such as quantum dots as secondary converters to produce white light. “Quantum Dots Lend New Approach to Solid-State Lighting,” Sandia National Laboratory press release Jul. 24, 2003. This approach incorporates quantum dots into a polymer used to encapsulate the light emitting diode (LED) and essentially creates a three-dimensional dome of quantum dots around the LED die. While this method has been successful in producing white light, the three-dimensional dome structure places large quantities of quantum dots in non-optimal positions around the LED and creates potential quantum dot agglomeration issues.
In general, SSL devices can be classified as either proximate or remote phosphor configurations depending upon the proximity of the phosphor and LED (see E. Fred Schubert, Light-Emitting Diodes, Second Edition, Cambridge University Press, 2006). In the proximate phosphor configuration, the phosphor and LED are contained in the same package and the phosphor often lies in intimate contact with the top surface of the LED. In the remote phosphor configuration, the phosphor and LED are physically separated to reduce the thermal efforts of the LED on the phosphor. The previous art described both proximate and remote phosphor configurations that suffer limitations overcome by the described invention. For example, U.S. Pat. No. 6,357,889 describes a color tunable light source utilizing a remote phosphor. This system requires at a minimum two different LEDs operating at two different wavelengths in order to produce white light. The increase complexity of this system results in increased costs. In another example, U.S. Pat. No. 7,144,131 describes an optical system consisting of a remote phosphor in which the phosphor is doped into a diffuse reflective material. Such a structure creates an optical system limited by the highly reflective properties of the host matrix, which requires that the entire structure contain phosphors, which increases costs. In another example, U.S. Patent Application 2009/0251884 describes an optical system consistent of a remote phosphor in which the phosphor is doped into a diffuse reflective material and the combined structure occupies the entirety of an optical integrating cavity. Such a structure also requires large amounts of phosphors increasing costs. Likewise, in such structures the luminescent material is integrated in a relatively permanent or complicated component that is expensive to replace, limiting the possibility of conveniently and inexpensively exchanging the carrier of the luminescent material for the purpose of maintenance or of allowing a variation of color.
Previously, polymer/quantum dot compound nanofibers have been obtained from electrospinning of the polymer/quantum dot composite solutions, as disclosed in Schlecht et. al., Chem. Mater. 2005, 17, 809-814. However, the nanofibers produced by Schlecht et al. were on the order of 10-20 nm in diameter, in order to produce quantum confinement effects. The size range of the nanoparticles and nanofibers disclosed therein is not advantageous for conversion of a primary light into secondary light emission across the white light spectrum.
Lu. et. al., Nanotechnology, 2005, 16, 2233, also reported the making of Ag2S nanoparticles embedded in polymer fiber matrices by electrospinning. Once again, the size range of the nanoparticles and nanofibers shown therein is not advantageous for conversion of a primary light into secondary light emission across the white light spectrum.
As described in U.S. application Ser. No. 11/559,260, filed on Nov. 13, 2006, entitled “LUMINESCENT DEVICE,” referenced above, highly-efficient, light-producing sheets have been developed based on a combination of photoluminescent particles and polymer nanofibers. These luminescent sheets can be used in a white-light solid-state lighting device in which the sheets are illuminated by a blue light-emitting diode (LED) light source and the sheets will transform the incident blue light into, for example, yellow light. An appropriate mixture of yellow and blue light will produce the appearance of white light.
One particular advantage of these light-producing sheets is that photoluminescent particles are suspended above the nanofibers instead of being contained in a bulk material with a relatively high index of refraction. This arrangement prevents light from being trapped by total internal reflection, as occurs when the nanoparticles are encapsulated within bulk materials.